1. Field of the Invention
The present invention relates to the packaging of integrated circuits and electronic components, and more particularly, to an improved underfill used in a flip-chip arrangement.
2. Description of the Related Art
It is known to form a microelectronic assembly by a direct chip attach method, commonly referred to as flip chip. An example of such microelectronic assembly is the assembly shown in FIGS. 1A and 1B, wherein an integrated circuit die 10 is mounted directly onto a supporting substrate 22. The supporting substrate 22 may be a printed circuit board or a package board with solder bump interconnections 12. The integrated circuit die 10 is spaced apart from the supporting substrate by a gap (not shown), and the solder bump interconnections 12 extend across the gap and connect bond pads 18 on the integrated circuit die 10 to bond pads 20 on the supporting substrate 22. In this manner, the integrated circuit die 10 is attached to the supporting substrate, and electrical signals are conducted to and from the die 10 for processing.
Because of differences in the coefficients of thermal expansion of the die 10 and the supporting substrate 22, stresses are created when the assembly is subjected to thermal cycling of the type experienced during operation. These stresses tend to fatigue the solder bump interconnections 12 and can lead to failure of the assembly. In order to strengthen the solder joints without affecting the electrical connection, the gap is filled with a polymeric underfill material 14.
The underfill material 14 is typically applied after the die 10 is attached by the solder bump interconnections 12 to the supporting substrate 22. A curable polymeric underfill material is dispensed onto the supporting substrate 22 adjacent to the die 10 and is drawn into the gap by capillary action. An additional amount of the underfill material is applied along the edges of the die 10 so as to form a uniform fillet 16 that extends beyond the edge of the die 10 and at least partially up the side of the die 10. The polymeric material is then cured, typically by heating, to form the underfill. The underfill bonds to the die 10, the supporting substrate and the solder bumps thereby strengthens the assembly and protects the solder bump interconnections from environmental damage.
The curing process, however, creates thermal stresses during the heating and cooling of the assembly. Mechanical stresses are also experienced during use of the microelectronic assembly, particularly in portable applications such as cell phones, PDA""s, etc. These stresses are normally detrimental to the die and the solder bump interconnections and can cause a detrimental warping of the integrated circuit die. The stresses also lead to cracking of the underfill. As shown in exemplary FIG. 2, the cracking 30 of the underfill commences from fillet 16 and a comer 32 of a die and propagates, with a tremendous force concentrated at the tip of the crack, through the solder bumps which are tightly bonded with the underfill.
Therefore, a need exists to reduce the detrimental effects of thermally induced stresses upon an underfill and the solder bump interconnections of a microelectronic assembly. Further, a need exists to strengthen and improve the reliability of a microelectronic assembly that includes reducing cracks in the underfill to improve the reliability of electrical intrconnections and to extend the useful life of an underfilled flip-chip arrangement.
There is, therefore, a need in the art of flip-chip integrated circuit manufacturing for a microelectronic assembly that maintains the proper physical structural integrity and electrical interconnections. Particularly, there is a need to control and reduce the undesirable effect of underfill cracking resulting from thermal stresses caused by, for example, thermal cyclings of an underfilled semiconductor die.
The present invention relates to a fillet portion of an polymeric underfill being softened or compliant to reduce occurences of crackings that may propagate through the underfill causing undesirable effects. Embodiments of the present invention comprise a semiconductor flip-chip arrangement having a semiconductor die with an array of conductive terminals coupled to an array of conductive pads on a supporting surface by solder bumps, including: a polymeric bonding underfill material disposed between the semiconductor die, and the polymeric bonding material further disposed along the edges of the semiconductor die forming a fillet of underfill material surrounding the edges of the semiconductor die, wherein the fillet is made substantially compliant so as to reduce occurences of crackings in the underfill material.
A further aspect of the invention is a method for manufacturing a flip-chip arrangement with flip-chip underfill, including the steps of attaching a semiconductor die to a substrate, underfilling the die, wherein a fillet is created at the perimeter of the semiconductor die, hardening the underfill material including the fillet, and softening only the fillet of the underfill.
Another aspect of the invention is a method for reducing fillet cracking in cured polymeric flip-chip underfill, which include the steps of applying a solvent to the fillet portion of the underfill to soften at least a portion of the fillet, and removing the solvent when the portion of the fillet is softened.
Another aspect of the invention is a method for reducing the occurences of cracking in a fillet in cured polymeric flip-chip underfill, including the steps of determining the location of fillet along a perimeter of a flip-chip, and irradiating the fillet with a thermal radiation source so as to apply thermal energy to the cured polymeric material sufficiently to break cross-links in the cured polymeric material to thereby render the fillet compliant.
The present invention provides significant advantages by reducing occurences of cracking in the underfill of a semiconductor die by rendering the fillet portion of the underfill compliant or soft so as to better absorb or dampen thermal stresses.
Another advantage of the present invention is that the softening of the underfill""s fillet can be accomplished by using a conventional chip washing machine loaded with an appropriate solvent and programmed for desired spraying, soaking, rinsing and drying cycles.
Another advantage of the present invention is the high throughput in the softening process of the fillet that can be accomplished using a thermal radiation source, particular a laser.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.